Historically the initiation and termination of meals has been attributed to endongenous factors, e.g. gastric load, levels of circulating metabolites, etc. Recent studies in this laboratory have suggested that exogenous, environmental factors may play an important role. In particular changes in the availability and nutritional quality have been shown to be important determinants of feeding patterns. For example, decreased availability leads to a reduced frequency of meals but of larger size conserving total intake; increasing the amounts of behavior required between bites leads to increased rates of responding conserving rate of intake. The response of animals of different species (cats, rats, and Guinea pigs in our case) to these variables is thought to reflect the ecological niche they occupy (carnivore, omnivore, herbivore). Constraints on these "natural" patterns of feeding have been shown to have very large motivational effects, resulting in response perserverance and response rates far beyond the obtained in deprived animals.